New York and London are famous for both their congestion and the diverse origins of their residents. But if you're looking for the ultimate teeming metropolis of immigrants, check out the large intestine. In people, some 500 to 1,000 kinds of bacteria reside in this part of the gastrointestinal (GI) tract, and these gut microbes outnumber all the cells in your body, perhaps by as much as a factor of 10.